Ink-jet printing is a non-impact printing process in which droplets of ink are deposited on print media, such as paper, transparency film, or textiles. Low cost and high quality of the output, combined with relatively noise-free operation, have made ink-jet printers a popular alternative to other types of printers used with computers. Essentially, ink-jet printing involves the ejection of fine droplets of ink onto print media in response to electrical signals generated by a microprocessor.
Ink jet printing systems generally are of two types: continuous stream and drop-on-demand. In continuous stream ink jet systems, ink is emitted in a continuous stream under pressure through at least one orifice or nozzle. The stream is perturbed, causing it to break up into droplets at a fixed distance from the orifice. At the break-up point, the droplets are charged in accordance with digital data signals and passed through an electrostatic field which adjusts the trajectory of each droplet in order to direct it to a gutter for recirculation or a specific location on a recording medium. In drop-on-demand systems, a droplet is expelled from an orifice directly to a position on a recording medium in accordance with digital data signals. A droplet is not formed or expelled unless it is to be placed on the recording medium.
Since drop-on-demand systems require no ink recovery, charging, or deflection, the system is much simpler than the continuous stream type. There are two types of drop-on-demand ink jet systems. One type of drop-on-demand system has as its major components an ink filled channel or passageway having a nozzle on one end and a piezoelectric transducer near the other end to produce pressure pulses. The other type of drop-on-demand system is known as thermal ink jet, or bubble jet. The major components of this type of drop-on-demand system are an ink filled channel having a nozzle on one end and a heat generating resistor near the nozzle. Printing signals representing digital information originate an electric current pulse in a resistive layer within each ink passageway near the orifice or nozzle, causing the ink in the immediate vicinity to evaporate almost instantaneously and create a bubble. The ink at the orifice is forced out as a propelled droplet as the bubble expands. When the hydrodynamic motion of the ink stops, the process is ready to start all over again.
Ink-jet inks are mostly available as dye based compositions. The main disadvantage of dye compositions (wherein the dye is dissolved in a solvent) is their limited lightfastness which results in dye fading. A very limited number of pigment-based inks are also available. Perhaps the desirable property that pigments offer is their superior lightfastness. However, pigments have not found extensive use in ink-jet ink compositions partly due to their natural tendency to agglomerate in aqueous media. Pigment particles tend to agglomerate because their outer surfaces create natural forces of attraction between the particles. The agglomeration of pigment particles can lead to nozzle clogging and result in poor print quality, due to defects such as those deriving from print density and mottling, and poor reliability.
Microemulsion based ink-jet ink is one of many ink-jet ink formulations which have been suggested in the past 20 years.
Several advantages are achieved with microemulsion based ink-jet inks. Microemulsion inks are highly stable solutions composed of finely divided droplets (10–100 nm) and low viscosity. Oil-in-water microemulsion based inks may provide fast drying times and excellent waterfastness of the drying film. In addition good wetting properties may be achieved on hydrophobic/hydrophilic surfaces.
U.S. Pat. No. 5,047,084 discloses a thermal ink jet ink composition in the form of a microemulsion including an organic vehicle phase having a colorant dispersed therein and an aqueous phase containing a surfactant, the vehicle phase preferably being liquid at 70° C. and solid at 20° C.
U.S. Pat. No. 5,531,816 discloses a pigment-based ink-jet ink compositions by formulating the ink compositions to include at least one appropriately modified pigment in a water-insoluble organic compound, which is microemulsified with an amphiphile and water. According to the invention the pigment is deagglomerated by means of encapsulation with a polymer, dispersion with a dispersing agent and surface modification.
U.S. Pat. No. 5,713,989 discloses aqueous pigment dispersion-based ink-jet ink compositions formulating the ink compositions to comprise at least one aqueous pigment dispersion and a microemulsion comprising at least one water-insoluble organic compound, at least one hydrotropic amphiphile, and water. The hydrotropic amphiphile is present in the microemulsion in an amount sufficient to solubilize the water-insoluble organic compound.
U.S. Pat. No. 5,749,952 discloses a thermal ink-jet ink composition containing a vehicle and a colorant, the vehicle comprises a microemulsion containing at least one substantially water-insoluble organic oil, at least one organic co-solvent, water and optionally at least one amphiphile and at least one high molecular weight colloid. The colorant is a water-insoluble chromophore that has been chemically modified to be water-soluble by addition of functional groups that impart water solubility. The inks described demonstrate high edge acuity, high optical density, fast drying times, reduced bleed, improved halo characteristics, high water fastness, and high smearfastness.
U.S. Pat. No. 5,342,440 discloses water-insoluble black dyes formulated in a microemulsion based ink. A combination of thermal ink-jet inks which avoid bleed between black and color inks was described.
U.S. Pat. No. 5,853,465 discloses an ink-jet ink set comprising: (a) microemulsion-based black ink in which the colorant is a water-insoluble black pigment that has been chemically modified to impart water solubility by addition of functional groups to form a macromolecular chromophore and (b) at least one non-black (cyan, yellow, or magenta) ink that is a typical aqueous dye-based ink with an agent that increases the ionic strength of the non-black ink and comprises either an inorganic salt or an organic acid. The organic acid is used to adjust the pH of the non-black ink to a value of less than 5. The non-black ink may alternatively comprise a water-insoluble non-black pigment that has also been chemically modified. The macromolecular chromophore-containing ink, or pigment-based ink, of the ink-jet ink set is a microemulsion that contains substantially water-insoluble organic oil, an organic co-solvent, water, an amphiphile and optionally a high molecular weight colloid. By employing an ink set in which some of the members of the set have a higher ionic strength than the black ink microemulsion-based ink, bleed alleviation can be achieved.
U.S. Pat. No. 5,226,957 discloses water-insoluble dyes formulated in microemulsion-based ink for thermal ink-jet printers, which are waterfastness, are non-threading, and are bleed alleviated. The ink-jet inks have a formula comprising: (a) about 0.05 to 0.75 wt % of a high molecular weight colloid; (b) about 0.1 to 40 wt % of at least two surfactants, (c) about 0.5 to 20 wt % of at least one co-solvent; (d) about 0.1 to 5 wt % of at least one water-insoluble dye; (e) about 0.1 to 20 wt % of an oil; and (f) the balance water. This invention uses a non-volatile oil phase due to evaporation associated with application in thermal ink-jet printers.
U.S. Pat. No. 5,492,559 discloses a thermal ink-jet ink composition that comprises an aqueous phase, an oil phase, an oil-soluble dye, and a surfactant, said ink exhibiting a liquid crystalline gel phase at a first temperature and a liquid microemulsion phase at a second temperature higher than the first temperature.
U.S. Pat. No. 5,643,357 discloses a thermal ink-jet ink composition which comprises water, an oil-soluble or alcohol-soluble dye and a surfactant, said ink exhibiting a liquid microemulsion phase at a first temperature and, at a second temperature higher than the first temperature, separating into a mixture of an aqueous liquid phase and a liquid crystalline gel phase.
U.S. Pat. No. 5,551,973 discloses a thermal ink-jet ink composition which comprises an aqueous phase, an oil phase, a photochromic material, and a surfactant, said ink exhibiting a liquid crystalline gel phase at a first temperature and a liquid microemulsion phase at a second temperature higher than the first temperature.
European patent No. 892025 discloses an aqueous microemulsion-based ink-jet ink composition that employs a second organic solvent that is miscible with both the discontinuous and the continuous phase. Specifically, the ink-jet ink composition of the invention comprises at least one dye and a microemulsion comprising at least one high-boiling water-insoluble organic compound, a second high-boiling compound miscible with both aqueous and organic phases, at least one amphiphile, and water, wherein the amphiphile is present in an amount sufficient to solubilize the water-insoluble organic compound. However, since a high boiling organic solvent in included in the compositions disclosed, when the composition is applied on a print media, the water first evaporates and a continuous hydrophobic film is formed which might lead to dye fading and smearing.
None of these prior art patents disclosed or suggested an ink based on oil-in-water microemulsion composition which is thermodynamically stable while in storage (in which the dye in dissolved in the composition) and wherein upon application of the ink to a substrate surface, the microemulsion converted into nanoparticles composed of the dye molecules, which have pigment-like properties. Such a composition will be highly advantageous because of its superior stability during storage and before jetting, will not cause clogging during jetting since no pigments are present prior to jetting and upon conversion of the microemulsion into nanoparticles (pigment) after jetting, good lightfastness and excellent waterfastness will be achieved.
While known compositions and processes are suitable for their intended purposes a need still remains for ink compositions suitable for ink-jet inks compositions for non-thermal, drop on-demand ink-jet applications such as Piezo, which employs piezoelectric element to pressurize and expel ink droplets onto print medium.
In addition there is a need for ink which is thermodynamically stable (as opposed to kinetically stable) while in storage, with “pigment like” properties upon application on substrate surface. Further, a need exists for ink composition which provide non-bleeding ink with fast drying times, improved lightfastness and excellent waterfastness. Additionally, there is a need for ink compositions that does not cause clogging of the print head. There is also a need for ink compositions that can be employed on low energy surfaces.